/*
 * Copyright (c) 2018-2019 Richard Braun.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * Upstream site with license notes :
 * http://git.sceen.net/rbraun/librbraun.git/
 */

#include <assert.h>
#include <errno.h>
#include <limits.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>

#include <kern/macros.h>
#include <kern/mbuf.h>

/*
 * Message header.
 *
 * The size denotes the size of the data, without the header.
 */
union mbuf_hdr {
    uint8_t size1;
    uint16_t size2;
    uint32_t size4;
#ifdef __LP64__
    uint64_t size8;
#endif /* __LP64__ */
};

static size_t
mbuf_compute_hdr_size(unsigned int order)
{
    return order / CHAR_BIT;
}

static void *
mbuf_hdr_get_msg_size_addr(union mbuf_hdr *hdr, unsigned int order)
{
    switch (order) {
    case 8:
        return &hdr->size1;
    case 16:
        return &hdr->size2;
    default:
#ifdef __LP64__
        return &hdr->size8;
    case 32:
#endif /* __LP64__ */
        return &hdr->size4;
    }
}

static size_t
mbuf_hdr_get_msg_size(const union mbuf_hdr *hdr, unsigned int order)
{
    switch (order) {
    case 8:
        return hdr->size1;
    case 16:
        return hdr->size2;
    default:
#ifdef __LP64__
        return hdr->size8;
    case 32:
#endif /* __LP64__ */
        return hdr->size4;
    }
}

static void
mbuf_hdr_init(union mbuf_hdr *hdr, unsigned int order, size_t size)
{
    switch (order) {
    case 8:
        assert(size <= UINT8_MAX);
        hdr->size1 = size;
        break;
    case 16:
        assert(size <= UINT16_MAX);
        hdr->size2 = size;
        break;
    default:
#ifdef __LP64__
        hdr->size8 = size;
        break;
    case 32:
        assert(size <= UINT32_MAX);
#endif /* __LP64__ */
        hdr->size4 = size;
    }
}

static size_t
mbuf_hdr_get_total_msg_size(const union mbuf_hdr *hdr, unsigned int order)
{
    return mbuf_compute_hdr_size(order) + mbuf_hdr_get_msg_size(hdr, order);
}

static unsigned int
mbuf_compute_order(size_t max_msg_size)
{
    unsigned int order;

    assert(max_msg_size != 0);
    order = (sizeof(max_msg_size) * CHAR_BIT) - __builtin_clzl(max_msg_size);
    return P2ROUND(order, CHAR_BIT);
}

void
mbuf_init(struct mbuf *mbuf, void *buf, size_t capacity, size_t max_msg_size)
{
    cbuf_init(&mbuf->cbuf, buf, capacity);
    mbuf->max_msg_size = max_msg_size;
    mbuf->order = mbuf_compute_order(max_msg_size);
}

void
mbuf_clear(struct mbuf *mbuf)
{
    return cbuf_clear(&mbuf->cbuf);
}

static void
mbuf_clear_old_msgs(struct mbuf *mbuf, size_t total_size)
{
    union mbuf_hdr hdr;
    void *msg_size_addr;
    size_t hdr_size;

    hdr_size = mbuf_compute_hdr_size(mbuf->order);
    msg_size_addr = mbuf_hdr_get_msg_size_addr(&hdr, mbuf->order);

    do {
        size_t msg_size, size;
        int error;

        size = hdr_size;
        error = cbuf_pop(&mbuf->cbuf, msg_size_addr, &size);
        assert(!error);

        if (size == 0) {
            break;
        }

        msg_size = mbuf_hdr_get_msg_size(&hdr, mbuf->order);
        size = msg_size;
        error = cbuf_pop(&mbuf->cbuf, NULL, &size);
        assert(!error && (size == msg_size));
    } while (cbuf_avail_size(&mbuf->cbuf) < total_size);
}

int
mbuf_push(struct mbuf *mbuf, const void *buf, size_t size, bool erase)
{
    union mbuf_hdr hdr;
    void *msg_size_addr;
    size_t hdr_size, total_size;
    int error;

    if (size > mbuf->max_msg_size) {
        return EINVAL;
    }

    mbuf_hdr_init(&hdr, mbuf->order, size);

    total_size = mbuf_hdr_get_total_msg_size(&hdr, mbuf->order);

    if (total_size > cbuf_avail_size(&mbuf->cbuf)) {
        if (!erase || (total_size > cbuf_capacity(&mbuf->cbuf))) {
            return EMSGSIZE;
        }

        mbuf_clear_old_msgs(mbuf, total_size);
    }

    hdr_size = mbuf_compute_hdr_size(mbuf->order);
    msg_size_addr = mbuf_hdr_get_msg_size_addr(&hdr, mbuf->order);

    error = cbuf_push(&mbuf->cbuf, msg_size_addr, hdr_size, erase);
    assert(!error);
    error = cbuf_push(&mbuf->cbuf, buf, size, erase);
    assert(!error);

    return 0;
}

int
mbuf_pop(struct mbuf *mbuf, void *buf, size_t *sizep)
{
    size_t start;
    int error;

    start = cbuf_start(&mbuf->cbuf);
    error = mbuf_read(mbuf, &start, buf, sizep);

    if (!error) {
        cbuf_set_start(&mbuf->cbuf, start);
    }

    return error;
}

int
mbuf_read(const struct mbuf *mbuf, size_t *indexp, void *buf, size_t *sizep)
{
    union mbuf_hdr hdr;
    void *msg_size_addr;
    size_t hdr_size, msg_size, size;
    int error;

    hdr_size = mbuf_compute_hdr_size(mbuf->order);
    msg_size_addr = mbuf_hdr_get_msg_size_addr(&hdr, mbuf->order);

    size = hdr_size;
    error = cbuf_read(&mbuf->cbuf, *indexp, msg_size_addr, &size);

    if (error) {
        return error;
    } else if (size == 0) {
        return EAGAIN;
    }

    assert(size == hdr_size);

    msg_size = mbuf_hdr_get_msg_size(&hdr, mbuf->order);

    if (msg_size > *sizep) {
        error = EMSGSIZE;
        goto out;
    }

    size = msg_size;
    error = cbuf_read(&mbuf->cbuf, *indexp + hdr_size, buf, &size);

    if (error) {
        goto out;
    }

    assert(size == msg_size);

    *indexp += hdr_size + size;

out:
    *sizep = msg_size;
    return error;
}